ELSEVIER Chemical Engineering and Processing 34 (1995) 323-328 Chemical Engme.ermg aria Processing On the dynamic simulation of mass crystallization with fines removal Matthias Kind, Ulrich Nieken BASK Technische Entwicklung, D-67056 Ludwigshafen, Germany Dedicated to Prof. Dr. Diei~mar Werner on the occasion of his 60th birthday Abstract In mass crystallization the size distribution of the crystals produced is determined by the rate of growth and nucleation. Both rates are functions of the supersaturation present in the system. Usually a low nucleation rate is preferred for producing large crystals, but this requires low production rates. Fines removal is an alternative means of achieving the desired coarse product. It is applied in tile industrial production of, for example, potassium chloride or ammonium sulfate• In this contribution the effect of fines removal, as well as the role of nucleation kinetics, has been investigated by dynamic simulation• The model used for simulation consists of the solute mass balance which is coupled by kinetics to the population balance of the crystals. The calculations show that true quantitative results cannot be obtained with simple models. Nevertheless, it is possible to investigate the dynamic behaviour of crystal size distributions. It is also shown how the intensity of fines removal and the presumed nucleation kinetics affect the stability of crystallizers. Keywords: Dynamic Simulation; Mass crystallization; Fines removal; Size distribution; Nucleation; Crystal growth 1. Introduction One important quality aspect of mass crystalline products is the crystal size distribution (CSD). The CSD is a strong function of the material characteristics• Some products crystallize readily in the 100-1.000/~m range, but often products hardly exceed 10/~m in size. The reason for this behaviour is that the solubility as well as nucleation and the growth kinetics of products can vary considerably [1]. The CSD can be modified by the selection of appro- priate operation parameters. For example, an increase in crystal size distribution requires low supersaturations and low abrasion rates. A second method of increasing the product size is the removal of fines during crystal- lization. This technology is applied in the preparation of numerous industrial products. Theoretical work on fines removal has been reviewed thoroughly by Moyers Based on a presentation by M. Kind and U. Niekan on GVC- Fachausschul3 'KristalIisation' in Wolfenbtittel, 11-I3 April 1994. and Randolph [2] and investigated experimentally by Juzaszek and Larson [3]. Fines which have been sepa- rated from crystallizer slurry are either dissolved by the creation of undersaturation conditions or they are com- pletely withdrawn. In practice, the separation of fines is achieved by means of settling zones within the crystal- lizer. The design of a crystallizer suited for fines with- drawal and the principal quantities involved are given in Fig. 1. The simulation of crystal size distribution is under investigation by a number of research groups at present [4-6], with the aim of primarily controlling the crystal size distribution arising from crystallizers. Therefore simulation calculations as presented in ihis paper are made to achieve a better understanding of phenomena observed in industrial crystallizers• The cal- culations seek to elucidate the complex interrelation of kinetics, operation parameters and fines removal, and demonstrate how they influence the resulting CSD. In this paper simulations are presented involving (i) varia- tion of the parameters involved in fines dissolution and (ii) variation of the nucleation kinetics.